1. Field of Invention
The present invention relates generally to improved methods for producing stable dispersions of nanoparticles and, more particularly, but not by way of limitation, to a single step milling and surface coating process for producing a stable dispersion of surface coated metal oxide nanoparticles.
2. Background of the Invention
Inorganic oxides such as titanium oxide and zinc oxide are often incorporated in cosmetics, paint, and plastics as whiteners, opacifiers, tinting agents, and the like. Particles of titanium dioxide, zinc oxide, and the like can also be used as an anti-UV agent in numerous applications, particularly in cosmetics, paint and plastics.
When used as a pigment, the performance of the particles involves absorption, reflection and scattering of visible light, which depends in large part on the particle size. For opacification, the optimum particle size of titanium dioxide is about 250 nm. When particles of titanium dioxide, zinc oxide, and the like are used as anti-UV agents, the performance involves absorption, reflection and scattering of the harmful UV radiation, and again depends to a large extent on the particle size. For example, titanium dioxide absorbs light with a wavelength of 405 nm or shorter. However, titanium dioxide also has a very high refractive index and is thus very effective in scattering. There is evidence that submicron titanium dioxide attenuates UVB (with a wavelength of from 290 to 320 nm) predominantly by absorption, while UVA (with a wavelength of from 320 to 400 nm) is attenuated predominantly by scattering.
There is a need for particle dispersions which are completely transparent, free from whitening when applied on the skin, but which still possess UV screening capabilities. It is known that when particles are smaller than one-half the wavelength of visible light, the particles will appear to form a transparent solution when completely dispersed. Thus, as anti-UV agents, the particles of titanium dioxide, zinc oxide, and the like desirably comprise a stable dispersion of nanoparticles. Nanoparticles generally refer to particles having at least one dimension of about 100 nanometers or less. Nanoparticles, unlike pigment size particles, scatter UVB light and UVA light more than the longer, visible wavelengths, and can thus prevent sunburn while remaining transparent on the skin. However, prior art processes for producing the nanoparticles are typically quite expensive and the dispersions produced are not stable.
For example, pigment particles are produced in a high-temperature reactor and then surface treated with metal silicates, dried, and further micronized to reduce particle agglomeration. Similar processes are available for producing nanoparticles which can potentially offer the desired transparency. For instance, the synthesis of nanoparticles of metals and mixed metal oxides through high-temperature oxidation of reactive precursors in oxygen plasma has been known for some time. However, plasma processing leads to agglomeration of the nanoparticles, which detracts from their desirability, especially where nanoparticle performance is required. Additionally, the high costs associated with plasma processing leads to costly end products and further limits their commercial attractiveness.
Ultra fine grinding techniques have also been investigated. As the particles become very small, their total surface area and surface energy become quite large, resulting in very high stress requirements for further fracture of the particles. Traditionally, as the particles decrease in size, the particles begin to flocculate or coagulate in order to decrease the total surface energy. Eventually particle size reduction approaches a limit and maximum energy is expended. Thus there is a need for lower cost, lower energy processes for producing metal oxide nanoparticles without aggregation and agglomeration.
Because titanium dioxide and zinc oxide are photoactive, i.e., free radical generators, to be effective in ultraviolet (UV) attenuation applications it is desirable to surface treat the titanium dioxide and zinc oxide nanoparticles to minimize or eliminate such activity. Particle absorption of a photon can result in production of a hole and an electron which can migrate to the surface of the particle and, in aqueous environments, form superoxide and hydroxyl radicals. It is known that a coating can capture these radicals and thereby reduce the apparent photoactivity.
Known processes for producing coated titanium dioxide particles typically include wet processing of particles that have been formed by plasma processing, ultrafine grinding or precipitation. During wet processing the particles are filtered, an aqueous slurry of the titanium dioxide particles is prepared, and the slurry is then treated with a metal precursor or salt to precipitate a metal oxide or hydroxide on the particle surfaces. Generally these surface coatings tend to cause additional agglomeration of the particles such that the coated titania must again be filtered, dried, re-ground to reduce agglomeration, and then redispersed. Although these coating methods can be used, the methods involve multiple energy-intensive steps.
Thus there are continuing needs for improved processes for making and coating metal oxide nanoparticles and producing stable dispersions therefrom.